As wireless cellular networks continuously evolve, orthogonal multiple access technologies such as a Code Division Multiple Access (CDMA) technology and an Orthogonal Frequency Division Multiple Access (OFDMA) technology that are widely applied to third-generation (3G) and fourth-generation (4G) mobile communications systems already gradually become inadequate to meet increasing capacity requirements of people on cellular networks, for example, are already inadequate to meet massive access and continuous increase of spectral efficiency, and the like. With ongoing development of research and application of non-orthogonal multiple access technologies, people hope that a future wireless cellular network such as a fifth-generation (5G) mobile communications system can effectively address a problem of increasing capacity requirements by means of non-orthogonal multiple access technologies.
In another aspect, conventional request-grant-based uplink data transmission in a cellular network is usually performed according to the following steps: First, a user sends a service request to a base station by using a specific resource (for example, a time-frequency resource). After receiving the service request, the base station grants uplink data transmission for the user according to a data buffer status reported by the user periodically or non-periodically, and delivers, to the user, an allocated resource for use in uplink transmission. Finally, the user transmits uplink data according to grant information by using the allocated uplink resource.
At an initial stage of an evolution process of a cellular network, a quantity of terminals increases relatively slowly, and users have relatively low requirements on delays. A conventional request-grant-based uplink data transmission method can be widely applied to 3G and 4G systems. However, as application scenarios, terminal types, and application types become increasingly varied, in a future evolution process of the cellular network, the quantity of terminals increases in an explosive manner. In specific application scenarios, users also impose higher requirements on network delays. In such a case, the conventional request-grant-based uplink data transmission method becomes no longer applicable because of relatively long delays and relatively high signaling overheads.
Compared with a conventional request-grant-based transmission method, in a non-grant-mode transmission method, a user directly sends uplink data by using a specific resource without needing to undergo a process from service requesting to uplink granting by a base station. Therefore, the non-grant-mode transmission method has significant advantages in terms of network delays and signaling overheads. In a non-orthogonal multiple access technology, it is allowed to use different codebooks to send different data streams on a same time-frequency resource, and a receive end can implement error-free decoding of multiple data streams. Therefore, a non-grant-mode uplink transmission method in which a non-orthogonal multiple access technology is combined has the potential to be extremely widely applied in future cellular communications systems (for example, 5G).
Currently, a non-grant-mode uplink transmission method in an SCMA system is: A base station assigns one contention transmission unit (CTU) to each user. The CTU is defined as a combination of a time-frequency resource and an SCMA codebook or a pilot sequence. After a user has reached uplink synchronization with the base station, if uplink data needs to be sent, the user directly generates the uplink data by using an SCMA codebook in a corresponding CTU and directly generates a pilot by using a pilot sequence in the corresponding CTU, and sends the uplink data and the pilot on a time-frequency resource specified by the CTU. The base station decodes user data on a possible time-frequency resource by using a blind detection method and by using a possible SCMA codebook and pilot sequence.
However, because the base station needs to try sizes of all possible transport blocks during decoding, so as to perform blind detection on user data. The blind detection has huge costs. For example, a processing delay is extremely long, and even exceeds a transmission delay that can be saved in a non-grant-mode transmission method, thereby causing the non-grant-mode transmission method to lose an advantage of a short delay over the conventional request-grant-based transmission method.